1. Field of the Invention
This invention generally relates to electronic circuitry and, more particularly, to linear source follower and emitter follower amplifiers.
2. Description of the Related Art
As noted in Wikipedia, a common-collector amplifier (also known as an emitter follower or BJT voltage follower) is one of three basic single-stage bipolar junction transistor (BJT) amplifier topologies, typically used as a voltage buffer. In this circuit the base terminal of the transistor serves as the input, the emitter is the output, and the collector is common to both (for example, it may be tied to ground reference or a power supply rail), hence its name. The analogous field-effect transistor circuit is the common-drain amplifier.
FIG. 1 is a schematic diagram of an emitter follower amplifier (prior art). The circuit can be explained by viewing the transistor as being under the control of negative feedback. From this viewpoint, a common-collector stage is an amplifier with full series negative feedback. In this configuration (with the gain β=1), the entire output voltage VOUT is placed contrary and in series with the input voltage VIN. Thus the two voltages are subtracted according to Kirchhoff's voltage law (KVL), the subtractor from the function block diagram is implemented just by the input loop) and their difference Vdiff=VIN−IOUT is applied to the base-emitter junction. The transistor monitors continuously Vdiff and adjusts its emitter voltage almost equal (less VBEO) to the input voltage by passing the according collector current through the emitter resistor RE. As a result, the output voltage follows the input voltage variations from VBEO up to the V+ supply voltage; hence the name, emitter follower. Intuitively, this behavior can be also understood by realizing that the base-emitter voltage in the bipolar transistor is very insensitive to bias changes, so any change in base voltage is transmitted (to good approximation) directly to the emitter. It depends slightly on various disturbances (transistor tolerances, temperature variations, load resistance, collector resistor if it is added, etc.) since the transistor reacts to these disturbances and restores the equilibrium. It never saturates even if the input voltage reaches the positive rail.
The common collector circuit can be shown mathematically to have a voltage gain of almost unity:Av=VOut/Vin≈0
A small voltage change on the input terminal will be replicated at the output (depending slightly on the transistor's gain and the value of the load resistance; see gain formula below). This circuit is useful because it has a large input impedance, so it will not load down the previous circuit:rin≈β0RE 
and a small output impedance, so it can drive low-resistance loads:rout≈RE∥Rsource/β0 
Typically, the emitter resistor is significantly larger and can be removed from the equation:rout≈Rsource/β0 
The amplifier functions as a voltage buffer. In other words, the circuit has current gain (which depends largely on the hFE of the transistor) instead of voltage gain. A small change to the input current results in much larger change in the output current supplied to the output load.
FIG. 2 is a schematic diagram of a source follower amplifier (prior art). The basic source follower has two problems. A change in input signal swing causes the Vds of M1 to change. The transistor has a finite gds that is a function of Vds and causes nonlinearity with swing. In order to provide more current to load, the current I needs to be increased, which causes increase in power dissipation.
FIG. 3 is a schematic diagram of a source follower amplifier with a constant Vds (prior art). The first of the above-mentioned problems is solved by using transistor M2 in parallel to M1, which keeps Vds across M1 constant and eliminates non-linearity due to gds of M1.
FIG. 4 is a schematic diagram of a power efficient source follower with adaptive biasing (prior art). The second of the above-mentioned problems is solved by using a current feedback loop to provide N·I current to the load, hence, improving settling bandwidth and nonlinearity performance at same power compared to architecture in FIG. 2. However this architecture still has limitations due to finite gds of M1.
It would be advantageous if a source follower amplifier could be designed that effectively addressed the problems associated with a non-constant Vds, power efficiency, and finite gain.